The present invention relates generally to ultrasonic distance measuring apparatus of the type which provides a distance output signal related to the time required for an ultrasonic wave to travel from a transmitter to a receiver and, more particularly, to such apparatus for use on a fork lift truck to determine the height to which the forks are raised.
Various measuring devices in the past have been used to measure distance by means of ultrasonic wave energy. If the speed of sound in the medium through which the ultrasonic acoustic wave passes is known, the time period required for an acoustic wave to travel between an acoustic energy transmitter and an acoustic energy receiver is proportional to the distance therebetween. Such acoustic wave systems have been utilized in numerous applications including, for example, measurement of the length or thickness of a pipe or other workpiece. In many such systems, the transmitter and receiver are positioned adjacent each other with the transmitted ultrasonic wave passing through the material and being reflected from the opposite surface of the material. In such an arrangement, as shown in U.S. Pat. No. 3,554,013, issued Jan. 12, 1971, to Berg, the transit time for the ultrasonic wave energy is twice the thickness or length of the material being measured.
It is desirable to provide an arrangement which senses the height to which the forks of a fork lift truck are raised. Such a fork lift truck typically includes an extendable mast upon which a pair of forks are mounted. By operation of truck controls, the operator may lower or raise the forks.
The operator of the fork lift truck may wish to insert the forks beneath a skid or container resting on the second or third level of a container storage rack. In such an instance, the operator knows the height of the container and, therefore, knows the height to which the forks must be raised. Previous height sensing arrangements, such as shown in U.S. Pat. No. 3,319,816, issued May 16, 1967, to Christenson, have incorporated electromechanical switching arrangements in which circuit resistances are varied in dependence upon the height to which the forks are raised, with a comparison being made with resistances indicating the desired height of the forks.
A similar circuit arrangement is shown in U.S. Pat. No. 2,790,513, issued Apr. 30, 1957, to Draxler. The Draxler circuit senses when the forks are raised above a predetermined level to limit the application of power to the drive motor of the truck, thereby limiting, to a degree, the maximum speed obtainable by the truck with the forks raised. Such electromechanical height sensing arrangements generally are relatively inaccurate due to variations in the resistors and in the mechanical linkages connected to the resistors.
While ultrasonic distance measurement apparatus would provide substantially increased accuracy in determining fork height, the adverse environmental conditions in which lift trucks operate, including extraneous acoustic noise and vibration and dust and other foreign matter, have heretofore precluded the use of such an ultrasonic distance measuring arrangement on a fork lift truck for measuring fork height.
Accordingly, it is seen that there is a need for simple, reliable ultrasonic height measuring apparatus for use on a fork lift truck to measure the height of the forks, which apparatus is capable of operation under adverse environmental conditions.